Saturday, August 13, 2011

Laboratory diagnosis of Respiratory Viral Infection

nullnullA. Respiratory syncytial virus:
• Collection of Specimens , virus isolation and cell culture
The gold standard method is isolation of virus in cell culture. RSV replicates optimally in HEP-2 cells. Specimens are best obtained by aspiration, gentle washing out of nasal, nasopharyngeal secretions or nasal swab (McCarthy and Hall, 1998). RSV is fragile and labile virus and may not remain viable if specimen transport is delayed. Once inoculated into cell culture, it replicates slowly with late development of CPE isolation time range from 3-10 days (Tristram, 2003). Shell vial culture improves RSV recovery and shortens isolation time to 2 days (Dunn et al., 2004).
• Detection of viral antigen
Detection of RSV Ag in respiratory secretions by direct fluorescent antibody (DFA) staining or by EIA is as sensitive as or superior to culture and result availability is clinically relevant and rapid tests have become the definitive practical methods for RSV diagnosis (Ohm-Smith et al., 2004).
• Serology
Serologic methods are used less frequently. In older children and adults, an increase in CF, neutralizing or ELISA serum antibodies (Abs) are a fairly sensitive index of reinfection with RSV. Asymptomatic infections are usually undetectable serologically. Serologic tests in infants are less sensitive, particularly in those younger than 4 months, with ELISA being the most sensitive assay. Serologic assays have been improved using ELISA with purified viral proteins or synthetic peptides (Wright et al., 2000).
• Molecular detection
RT-PCR has superior sensitivity over all other methods and is more reliable